The present invention relates generally to a device for purifying feedstock to be used in fuel cells, and more particularly to material and configuration improvements in a methanation reactor to facilitate carbon monoxide removal from a reformate stream.
While conventional power source devices (such as internal combustion engines, including piston and gas turbine-based platforms) are well-known as ways to produce, among other things, motive, heat and electric power, recent concerns about the effects they and their fuel sources have on the environment have led to the development of alternative means of producing such power. The interest in fuel cells is in response to these and other concerns. One form of fuel cell, called the proton exchange membrane (PEM) fuel cell, has shown particular promise for vehicular and related mobile applications. A typical PEM construction includes an anode and a cathode, with a solid polymer electrolyte membrane spaced between them such that protons generated at the anode can travel through the electrolyte and to the cathode. In PEM fuel cells, hydrogen or a hydrogen-rich gas is supplied to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied to the cathode side of the fuel cell. Catalysts, typically in the form of a noble metal (such as platinum), are placed at the anode and cathode to facilitate the ionization of hydrogen and subsequent reaction between it and oxygen.
In an ideal fuel supply situation, pure hydrogen (H2) gas is used as a direct fuel source. This is impractical in many vehicle-based fuel cell systems, as the amount of gaseous hydrogen required to be carried in order to achieve adequate vehicle range between refueling stops would be prohibitively large. A promising alternative to the direct feeding of H2 is the reformation of on-board liquid hydrocarbons through a fuel processing system upstream of the fuel cell such that the liquid hydrocarbons are converted into H2-rich feedstock. Methanol (CH3OH) is an example of a readily-available hydrocarbon fuel, and accordingly has become one of the preferred H2 precursors, especially for volume-constrained mobile fuel cell applications. Its relative low cost and liquid state at practical temperatures of interest make it compatible with existing fuel delivery infrastructure. Unfortunately, during the conversion of CH3OH to H2, carbon monoxide (CO) is also produced, of which even minute amounts can poison the noble metal catalyst on the downstream fuel cell anode and cathode. While reformation of CH3OH often produces only small amounts of CO, even such small quantities have a deleterious effect on fuel cell power output and life. Hence, the CO concentration needs to be attenuated to very low levels (preferably below 10 ppm) for the H2-rich reformate to be suitable as fuel cell feedstock.
A typical fuel processing system incorporating CH3OH includes a reformer and one or more purification (or clean-up) stages. There have emerged three general types of reformers that can be used on CH3OH and related liquid hydrocarbons: (1) steam reforming; (2) partial oxidation reforming; and (3) autothermal reforming. In the first variant, a pre-heated mixture of fuel and steam is reacted, while in the second variant, a pre-heated mixture of fuel and air is reacted. The third variant combines elements of both processes in a single reactor, and using a specially designed catalyst, enables balancing of the endothermic first and exothermic second variants. In all three cases, a reformate containing the desired end product, gaseous H2, as well as undesirable CO, is produced. A shift reactor may be employed to convert the CO in the reformate with water into CO2 and H2 in what is called a water-gas shift reaction. Since the water-gas shift reaction is reversible, it has been found that to promote the formation of CO2 and H2, the reformate should be cooled. Serially connected shift reactors of successively lower temperatures may be used to further reduce the CO concentration. While this level of CO cleanup could be sufficient for certain types of fuel cells, it is still not adequate for others, such as PEM fuel cells. While much of the present disclosure is in the context of PEM fuel cells, it will be appreciated by those skilled in the art that the invention disclosed herein has utility in other forms of fuel cells, where clean-up of fuel precursors can be used for improved fuel cell system operability, as well as for other processes where highly purified H2 feedstock is necessary. Accordingly, at least for PEM fuel cells, additional steps must be taken to ensure that the concentration of CO in the reformate is further reduced. There are numerous ways to provide such reduction, including preferential oxidation of CO, the use of diffusion membranes to separate H2 from the CO, and catalytic methanation reactions. Often, two or more of these methods can be used in combination to achieve the exceptionally low CO concentrations necessary for proper PEM operation. Of these, the methanation reaction is achieved by reacting CO with some of the just-produced H2, typically in the presence of a catalyst, to produce methane (CH4) and water according to the following:CO+3H2→CH4+H2O  (1)Typically, the reformate stream contains (in addition to the CO) other by-products, most notably CO2. Accordingly, methanation reaction (1) above must compete with the following:CO2+4H2→CH4+2H2O.  (2)Typical reformate streams, including those produced by the reformation of CH3OH, can possess considerably higher concentrations of CO2 than CO, often more than an order of magnitude higher. As a potential methanation reaction, reaction (2) is particularly undesirable because, given the relatively high concentration of CO2 in a CH3OH reformate stream, it rapidly reduces the amount of H2 available to the fuel cell anode (by consuming four H2 molecules for every CO2 molecule), thereby leaving the lower concentration and more poisonous CO relatively unreacted. In addition, if the concentration of CO2 is relatively high (on the order of a percent or more), the strong affinity for H2 cuts into the available H2 fuel supply, significantly reducing fuel efficiency. Thus, to minimize both H2 losses and the presence of poisons being delivered to the fuel cell anode, methanation reactions should be used when the concentration of both of the aforementioned carbon oxides is relatively low. Such a situation is most readily achieved when the methantion reactor works in conjunction with one or more of the previously indicated other clean-up devices.
One way to promote the reaction (1) to the exclusion (or near exclusion) of reaction (2) is to employ some means for selective methanation. Selective methanation involves the preferential reaction of one reactant species in lieu of others when others are also present. In the present case, it is desirable to achieve the selective methanation of CO over CO2 to remove the more harmful former from the reformate stream that will eventually find its way to the fuel cell anode. Previous attempts at selective methanation of CO over CO2 have proven to be too difficult, and hence expensive, to be viable for large-scale commercial use. One reason is that the disproportionate concentration of CO2 to CO, in addition to leading to the aforementioned inordinate consumption of H2, also leaves some of the CO unreacted. While some of this preferential reaction with CO2 can be meliorated with proper methanation catalyst choice, such choices are limited because of the second reason: both reactions (1) and (2) are exothermic (heat-producing) in nature, thereby producing higher temperatures in the region around the methanation catalyst. These high temperatures remove from consideration the relatively limited number of catalysts (for example, rhodium and ruthenium) that are good at promoting selective methanation of CO over CO2, as these catalyst function best at low temperatures. In addition, high methanation temperatures can facilitate the aforementioned reverse water gas shift reaction, which by producing more CO, is undesirable. Supplemental thermal management schemes, such as intrusive heat exchangers and coolant injection can be used, but such schemes increase system weight, volume, cost and complexity, especially those which are untenable in configuration where space and weight come at a premium, such as in vehicular and related mobile applications. On the other hand, the temperature must also be high enough to promote adequate levels of methanation activity. This is important, as CO conversion (i.e., activity) is strongly temperature-dependent, where higher temperature regimes tend to promote CO methanation better than lower temperature regimes. Accordingly, existing methanation reactors, typically with the aforementioned rhodium or ruthenium catalysts on an alumina (Al2O3) support, are neither sufficiently reactive at lower temperatures nor sufficiently selective at higher temperatures to maximize conversion of CO to CH4.
Accordingly, there exists a need for a methantion reactor that can achieve selective methanation of CO over CO2 (and other competing species) without having to resort to approaches that require significant increases in weight, volume or complexity. There also exists a need for a methanation reactor that is compatible with other reformate stream CO reduction approaches.